1. Field of the Invention
The present invention relates to an optical device, and more particularly, to a fiber-optic device capable of converting a polarization state in a single mode fiber.
2. Description of the Related Art
A technique for controlling polarization inside an optical fiber has been used for various fields, such as an optical communication field, an optical fiber laser, and an optical fiber gyroscope, and various methods of separating or converting a polarization state have been proposed.
For example, the following methods have been proposed or used: a method of using polarization-maintaining optical fibers having high birefringence or physically deforming the polarization-maintaining optical fiber to change a polarization state; a method of connecting one or more external polarizing elements and a bulk lens or a lens group to change a polarization state; a method of using a cylindrical piezoelectric element to make an optical fiber have birefringence, thereby changing a polarization state; a method of using the birefringence of an optical fiber grating imprinted on a high birefringence fiber to separate two orthogonal polarization state; and a method of polishing the side of an optical fiber and coating the optical fiber with a metallic material to separate a polarization state.
Most of the polarization-maintaining optical fibers have complicated structures. Therefore, the following methods are used in order to change a polarization state: a method of adding other structures (for example, a bow-tie structure or a Panda optical fiber) to the optical fiber having a core and a cladding to apply stress to the core, thereby inducing birefringence; and a method of manufacture an asymmetrical optical fiber core to induce birefringence. However, the polarization-maintaining optical fiber has a complicated structure in order to obtain birefringence, which makes it not easy to manufacture a high birefringence optical fiber. In addition, generally, there is no great connection loss when single-mode optical fibers are connected to each other, but it is difficult to connect different types of optical fibers.
External polarizing elements, such as a thin-film linear polarizer and a wave plate, are known as good polarizing elements. However, since these external polarizing elements need to use separate bulk lenses or lens groups, the manufacturing processes thereof become complicated, and these external polarizing elements are very expensive.
Next, the method of winding an optical fiber on a cylindrical piezoelectric element can adjust a voltage applied to the piezoelectric element to rapidly control the birefringence, and thus induce the difference between the phases of optical signals due to the birefringence. However, in this case, the size of the piezoelectric element needs to be large in order to minimize an optical loss due to the bending of the optical fiber, and a reinforcing member for compensating for birefringence modulation or polarization modulation should be additionally provided on the outside of the optical fiber that is wound around the piezoelectric element.
The method of using the birefringence of the optical fiber grating has a problem in that it is effective for only a specific wavelength due to the wavelength dependency of the optical fiber grating. In the method of polishing the side of an optical fiber and then coating the optical fiber, the optical fiber core having a very small diameter needs to be accurately polished and then coated with a metallic material, which results in a complicated manufacturing process. In addition, the two methods can control only the polarization states orthogonal to each other.
Therefore, a polarization converter capable of controlling the polarization state of an optical signal without using a polarization-maintaining optical fiber or an external polarizing element is needed.